The present invention relates to asphaltic compositions and, more particularly, to asphaltic compositions for use in soil stabilization and recycling of old asphalt pavements.
Historically, the early asphalt roads in the United States were built with Trinidad Lake asphalt, from the Island of Trinidad. After the discovery of asphaltic crude oils in this country, the residue, mostly asphalt, from the distillation of these oils was used for road oiling. Later, asphalts cut back with volatile solvents were mixed into or applied to the road surfaces as primers or to produce road pavements. Hot mix pavements came into use in which asphalt is heated and mixed with aggregate, and laid to produce a pavement. Also, asphalt emulsions were developed. At the present time, the ecological and energy drawbacks of hot mix and cut-back asphalts have brought asphalt emulsions to the front since they avoid the use of organic solvents and expensive hot mixing, thereby saving energy and reducing air pollution.
Selection of paving asphalts has traditionally been made on the basis of penetration, viscosity and ductility. Penetration, a well known test, is an indication of the hardness of an asphalt. In the past, the general assumption was that asphalts with a penetration below 35 at 77.degree. F. were too hard and brittle for use in paving. Softening point is another indication of quality which is hereinafter further discussed.
For years it has been realized that distilling crude oil by the prior conventional methods, including oxidation, to produce low penetration asphalt destroyed the ductility of the asphalt. However, after the introduction of solvent refining processes, it was eventually realized that asphalts could be produced with low penetration while preserving high ductility. U.S. Pat. No. 3,216,336 to Jewell R. Benson discloses a cut back penetrative binder for stabilizing soil and road surfaces. This binder includes a selected hard asphalt cement having a penetration less than 25 and even down to 0. The selected asphalt is usually a solvent refined or steam refined type and is characterized, and may be defined by relationship of hardness to softening point, as set forth in U.S. Pat. No. 3,216,336.
To gain the ecological advantages inherent in an emulsion, Mr. Benson used a small amount of volatile solvent in a hard base asphalt to permit the same to be emulsified, as disclosed in U.S. Pat. No. 3,399,608. Later, it was discovered that the hard base asphalt could be emulsified directly. Both products had the desirable property of imparting to pavements high compressive strength, far exceeding the strength of pavements made with comparable standard emulsions and even exceeding the strength of the more expensive hot mix pavements.
The limitations of Benson's products, were for priming and soil stabilization. His products, when cured, were deficient in ductility and cementing value. However, the developments by Benson were a step in the right direction toward stronger and more durable pavements.
Thus, there is still a need in the art for an asphalt composition which has the high strength of the selected low penetration asphalt; which can be easily emulsified; and which exhibits high ductility and low softening point so that, when emulsified and mixed with aggregate, it will produce pavements with greater load carrying capacities, greater water resistance and greater stability while at the same time retaining the desirable characteristics of flexibility. Only a very few naturally occurring asphalts have such desirable properties as more fully hereinafter set forth. One example is the Trinidad Lake asphalt. Other examples are certain special crudes in California, Iran and Canada. However, these materials are limited in supply and not generally available. There is a need for a blend which can be obtained nationwide and which has the desirable properties of such limited materials.